Other Parts Discussed in Thread: ADS1148
Hi,
I need to interface the below humidity sensor with ADS1148. The sensor provides both the humidity and temperature records. The output of the sensor is 4-20 ma. The sensor consumes up to 1 watts of power and about 40 ma of current. The sensor as seen is 2 wire type. I would be happy to provide more details if required.
Thanx,
Himanshu.V.Ajmera
Hello Himanshu,
I was assuming that you were already using bipolar +/-2.5Vsupplies from the posts submitted before this one. I am not sure I understood your question about the voltage requirement (10-20v). The sensor has an output current that can be scaled depending on the load resistor. The ADS1148 can only convert voltages that are inside its input voltage range which is limited both by the reference voltage and the AVDD/AVSS supplies.
The sensor has an output current for 4-20mA. If you decided to use the +/-2.5V bipolar supplies for this application, connecting a low drift, stable, precision resistor of 100 Ohms to GND should convert the 4-20mA output signal of the sensor to a voltage of 0.5-2V referred to GND. In this case, this signal could be converted by the ADS1148 with PGA gain =1 and using the 2.048V internal reference. In this case, you would connect in this AINN=GND and AINP to the resistor. The common-mode range requirements are met since you are using bipolar supplies.
If you plan to use a unipolar supply AVDD=5V, and AVSS=0V, you could possibly use a current to voltage converter and bias the common-mode voltage signal so the common-mode ADS1148 conditions are met...
You may find a discussion of the common-mode input voltage and full scale input requirements for the ADS1148 with examples on this link:
If you choose to have an external reference connected, the voltage of the VREFP and VREFN input pins must meet the following conditions:
For example, if you decided to use unipolar supply, where AVDD=5V and AVSS=GND; the easiest way to go is to connect VREFN to GND and VREFP to the output of the reference. In this case, the GND of the external reference will be connected to GND. Notice the maximum Reference voltage is limited by the supplies (AVDD-AVSS)-1V; therefore the maximum reference voltage you may use is 4V....
For the bipolar supply +/-2.5V case; you must ensure that all the voltage reference input conditions on the table above are met. If the voltage reference is greater that 2.5V; you will need to refer the external reference to AVSS=-2.5V in order to provide enough headroom. The REFN and REFP inputs will connect accross the voltage reference; so if the external reference is referred to AVSS=-2.5V; VREFN will have to be connected to AVSS also.
The key in both cases is that all the conditions on the table above for the reference inputs are met.
Regards,
Luis
Hi Himanshu,
The required connections of the sensor you provided are shown below; where the load resistor will be placed the ADS1148 board in close proximity to the converter. Notice how the load resistor is connected to GND on the ADS1148 board. The power supply for the sensor will also refer to the same GND. The sensor you provided requires a voltage above 10V :
V min = 10V + (0.020*Load);
So if the load resistor is 100 Ohms you require a voltage supply above 12V (maybe >12V is a good voltage). Each sensor consumes a max of 40mA. If you require 8 sensors; you need a supply that is able to provide at least 320mA for the sensors. If you decide to use a DC to DC converter from the 5Vdd supply that is connected to the ADS1148; you will need to carefully isolate the switching supply from the ADS1148.
The decision on how to connect all 8 sensors (16 outputs) depends more on your application requirements than the converter itself. The ADS1148 has eight inputs so you could connect 7 single-ended inputs to each. Another approach is that you could use the ADS1148's programable GPIO's to control an external 16 channel MUX to switch between all 16 channels using one ADS1148 (provided that your application allows for the time to switch between channels). Also, you may consider using a shielded sensor cable. See the 2 generic diagrams below.
Regards,
Luis
Hello Luis,
The terminal block diagram (diagram 1) as shown below consist of 4 IOs. As seen, two are for the input supply while 2 wire indicates T-out and H-out. Hence, basically there are separate 4-20mA outputs for T and H, while what I understood from the generic diagram posted above is same wire transmits 4-20mA current output for T as well as H. So I think there would be a slight modification- two resistance to be placed for each sensor.
The Vdd supply we are discussing is the AVdd supply right? And will the current from the 5V Vdd supply be sufficient to generate minimum 320mA through DC-DC converter? The efficiency of the converter would be around 85%, so will the 1Watt power requirement for each of the 8 sensors be contented? What will be a good design-to provide the supply for the sensors through Vdd or use some external source directly giving 10-15V?
I am posting my current configuration of one such sensor (diagram 2). I hope you get a clear idea as how I would want to interface the sensor with ADS1148.
My design would require to use 7 channels of ADS rather than using a multiplex and a single ADS. Only one thing here, can I directly ground one of the channel from ADS for single ended measurement?
The pins 1, 2, 3 etc are signal conditioning pins on the PLC itself.
Thanx,
Himanshu.V.Ajmera
